Pissart (2009, 2014) produced a map showing the distribution of "fish tanks", traces of lithalses, in Eastern Belgium (Hautes Fagnes) and he insisted on a close relationships with the systematic presence ... [more ▼]

Pissart (2009, 2014) produced a map showing the distribution of "fish tanks", traces of lithalses, in Eastern Belgium (Hautes Fagnes) and he insisted on a close relationships with the systematic presence in the subsoil of the Upper Cambrian rocks (former Revinian), namely: quartzites, quartzophyllades and phyllades. He made of this lithological aspect a fundamental factor, as well as the periglacial climatic conditions, to justify the formation of lithalses in Eastern Belgium. He points out one exception in the Elsenborn military camp, where the Lower Devonian sandstones and silty shales are exposed. The researchs that led to the realization of this map were carried out on the aerial photos made available to researchers and the public, from the 1950s, by the Military Geographical Institute (IGM). In 2015, the Walloon Public Service, through its "WalOnMap" site, has launched digital terrain models generated by LIDAR (Laser Detection and Ranging) images, including the "2013-2014 (Hillshade)" model, which only gives the relief of the ground, irrespective of the vegetation cover, and offers, by this advantage, a potential for prospecting. This one is clearly superior to that of the above-mentioned aerial photos. [less ▲]

Pissart (2010) published a synthetic map locating high "viviers" (fishponds) areas in the Ardennes (Belgium), some of which were identified in the 1950s' on aerial photographs taken by aircraft flying at ... [more ▼]

Pissart (2010) published a synthetic map locating high "viviers" (fishponds) areas in the Ardennes (Belgium), some of which were identified in the 1950s' on aerial photographs taken by aircraft flying at low altitude; others have been found on Google Earth in the 2000s'. However, be aware that one and the other source is usable for this purpose only in open landscapes. In 2015, the Public Service of Wallonia, via his site "WalOnMap" launches the Lidar mapping. Among other possibilities, this tool allows only restore the relief of the ground (digital field model), regardless of the plant cover (MNT 2013-2014 "Hillshade") . So the question is to estimate what can be the contribution of this new tool in the identification of traces of lithalsas. [less ▲]

It is generally held that, in north-western Europe, the main part of the gravel sheets under river beds were deposited during the Weichselian period in a periglacial environment. However, other parameters ... [more ▼]

It is generally held that, in north-western Europe, the main part of the gravel sheets under river beds were deposited during the Weichselian period in a periglacial environment. However, other parameters such as propagation of knickpoints in fluvial networks may also influence incision or aggradation. However, only few studies have dated the periods of formation of the gravel sheets and have described their properties. The first aim of this research was to determine the thickness of the gravel sheets still remaining under the river beds and to estimate the potential incision of these rivers before reaching the bedrock. Then we tried to answer a number of other questions: When did these thick gravel deposits fill the valley bottom? When were the lowest terraces abandoned? When did the rivers incise the bedrock? What is the morphology of the bedrock under the gravel layer? Numerous boreholes were made by percussion drilling in different floodplains of the Ardenne Massif and core samples were taken, down to the bedrock. Afterwards, different volcanic tephra from the Late Pleistocene were used as stratigraphic markers to date the relative periods of terrace formation and to reconstruct the past evolution of the gravel sheets. Pollen and metallurgic slag were also used to date the periods of bed level evolution. In the Ardennian massif, the thickness of the gravel sheet beneath the river beds is very variable (from 10 m in the downstream part of the Ourthe River to less than 1 m in the upper catchments). In some valleys, weathered bedrock has been observed under the gravel sheet to a thickness of several meters. Different phases of accumulation and incision over the last 100,000 years have been dated. Some evolutions can be clearly linked to climate changes but some modifications of bed levels also occurred during the Weichselian period and could be a response to the propagation of knickpoints in the fluvial networks. [less ▲]

The geomorphological dynamics (incision, lateral mobility, sedimentation rate) of a river typical of the Ardenne region (the Lienne River) were studied at the level of a valley floor peat deposit. This ... [more ▼]

The geomorphological dynamics (incision, lateral mobility, sedimentation rate) of a river typical of the Ardenne region (the Lienne River) were studied at the level of a valley floor peat deposit. This type of site is of major interest as the morphology of ancient fluvial deposits is preserved underneath peat deposits and pollen conserved in the peat allows different phases of the evolution of the river to be dated. The presence of pollen from the Younger Dryas above a pebble sheet perched 1.4 m above the present-day bed, as well as peat deposits from the Preboreal at the level of the present-day bed indicate that a phase of incision occurred during the Younger Dryas-Preboreal transition. Reworked Laacher See tephra was found in the upper part of the perched pebble sheet, which confirms that it had been laid down during the Younger Dryas, in a periglacial context. The Lienne River had probably multiple channels during the Preboreal. After the abandonment of one of the channels, it moved laterally, allowing the peat to spread until it occupied more than two-thirds of valley from the Atlantic phase. The use of slag from steel working as a stratigraphic marker shows low lateral mobility in the Lienne River as well as a low rate of aggradation of the alluvial plain over the last centuries. [less ▲]

Origin of megaliths of the Fond de Quarreux (Ardenne, Belgium). Distribution of megaliths was studied in the Ambleve valley throughout its gorge developed within the quartzites of the Cambrian-aged La ... [more ▼]

Origin of megaliths of the Fond de Quarreux (Ardenne, Belgium). Distribution of megaliths was studied in the Ambleve valley throughout its gorge developed within the quartzites of the Cambrian-aged La Venne Formation (Fond de Quarreux) in the south-eastern part of high Belgium. The high concentration of megaliths at this place can be essentially explained by mass movements which have occurred on the adjacent slopes in periglacial periods of cold Ocean Isotopic Stages in Quaternary times, and especially the coldest ones i.e. even O.I.S.#22 through #2. Stratigraphical studies of deposits in the Chefna and Ambleve valleys allowed us to place final periglacial deposits during the Younger Dryas. The critical shear stress and the critical unit stream power are used in order to evaluate the Ambleve River competence. It results that, under the present climatic conditions, this river cannot move boulders of more than 50 cm in diameter. Hence, it is concluded that ice rafting has been the main process responsible for carrying away the megaliths which have got the Ambleve river bed throughout the Quaternary Period. [less ▲]

In 2004, the Butgenbach lake reservoir (Warche river) has been emptied for the first time since 1932. The sediments deposited have been studied with a double objective : on the one hand to determine the ... [more ▼]

In 2004, the Butgenbach lake reservoir (Warche river) has been emptied for the first time since 1932. The sediments deposited have been studied with a double objective : on the one hand to determine the volume of sediment deposits, and on the other hand to estimate mean soil erosion rate in the catchment. Some five hundreds manual augerings were made using a thin auger (15 mm) throughout the wet and soft sediments (mud) in order to identify the lake deposits that have overlain the previous soft material (flood silts in the flood plain of the river Warche, silty soils on the slopes). Since the expected boundary between both types of materials could not be identified by the naked eye, a qualitative analysis of diatoms, magnetic susceptibility, grain-size distribution and calcination has been made for fifteen thicker cores (3.6 mm across) taken at representative sites. Mean annual area-specific sediment yield amounts to 28 – 33 t.km-².y-1 [less ▲]

The aim of this study was to estimate sedimentation rates in floodplains and to investigate lateral channel migrations over the last centuries. Considering the difficulties in determining these parameters ... [more ▼]

The aim of this study was to estimate sedimentation rates in floodplains and to investigate lateral channel migrations over the last centuries. Considering the difficulties in determining these parameters using traditional dating methods, we have used different kinds of slag rejected from former metal works. In the Ardenne region (southern part of Belgium), at the end of 14th century, iron factories moved near to rivers in order to take advantage of the hydraulic energy. At the same time, large quantities of slag produced in primitive blast furnaces were piled onto the floodplains, often very close to rivers. Consequently, microslag could be carried away either during floods or by lateral erosion to be deposited on the floodplains downstream. In order to use slag as a stratigraphical tracer, we located slag waste sites along rivers and we dated the periods of furnace activity from historical studies. Afterwards, we measured slag concentrations in the alluvium and we studied their evolutions in different borings. These results allowed us to reconstruct the appearance of the floodplain at the inception of the metal works and to estimate lateral channel mobility over several centuries. Moreover, different kind of tephra and 14C dating have been used in order to date the deposits that have been established before the settlement of metallurgy. [less ▲]